The present invention is directed to a humidity limiting mode of operation for heating, ventilating and air conditioning systems. More specifically, the humidity limiting mode of operation is initiated whenever relative humidity is greater than a relative humidity limit. The humidity limiting mode of operation will operate in the traditional deadband between the heating and cooling temperature setpoints, or will otherwise operate when the temperature versus the temperature setpoint is considered to meet the temperature control requirements. In addition to humidity, the humidity limiting mode of operation is controlled based on a zone condition such as temperature or dewpoint so as to integrate the temperature or dewpoint control with the humidity limiting control.
Dehumidification reduces relative humidity by removing water from air while maintaining a constant temperature. Relative humidity control is a broader control which includes dehumidification but also recognizes that the relative humidity can be limited by increasing air temperature even if the amount of water vapor in the air does not change.
When dehumidification was needed in previous systems, an air conditioning system was operated until a relative humidity was less than a relative humidity setpoint. Separately, the air conditioning temperature control maintained a temperature within a setpoint limit or limits. The dehumidification control and the temperature control were entirely distinct, but permitted either unwanted interactions in continuous-type controls or unwanted responses in humidistat-type controls. For instance, existing continuous controls allow both heating and cooling to be simultaneously called for. This allows the HVAC unit to operate in a dehumidification mode but potentially without effective dehumidification since the simultaneous heating and cooling cannot guarantee that moisture removal, dewpoint control or dehumidification is occurring particularly when cooling capacity operates at intermediate levels. An integrated control is desirable where the temperature and dehumidification controls are integrated, where the transitions between the modes of operation are smooth, and where the interactions between the temperature and humidification controls are managed. Some facets of the present invention include the use of equipment heating and cooling capacities appropriate to operating needs and at pre-determined initial operating levels when transitions between modes are required. This minimizes transient deviations from desired conditions and the resulting added energy expense.
More specifically, previous systems use a thermostat for cooling control with some peripheral dehumidification, and use a humidistat for dehumidification control with reheat added to avoid subcooling. The thermostatic reheat control normally operates when a zone heating load becomes apparent from the operating cooling capacity for dehumidification. The thermostat and the humidistat operate independently without any integrated control. In previous systems, there is no automatic selection of reducing relative humidity with heat versus reducing relative humidity with cooling; there is no smoothing of the transition between the modes of operation; and there is no maintenance of zone occupant comfort by minimizing zone temperature fluctuations. Generally, the air conditioning control in previous systems does not operate in the deadband between the heating setpoint and the cooling setpoint.
In the case of unoccupied buildings, the setpoint is often changed from an occupied setpoint to a more energy conservative unoccupied setpoint thereby activating the HVAC system less often. However, drift or lack of control in the deadband can allow the space temperature and relative humidity to move well away from the setpoint, at a significant later energy penalty.
In the present invention, control sequences implement the use of cooling and heating capacity according to both sensible temperature and relative humidity conditions. Integrated control of both parameters avoids several energy wasting conditions which are produced by non-coupled, single parameter dependent dehumidification control schemes. Energy wasting effects include use of heating and cooling capacity when cooling capacity is operating above dewpoint temperature which therefore do not dehumidify, and use of heating and cooling capacity when dry bulb zone conditions are not near the desired setpoint.